Two structures are widely used at present for optical receivers in general. A first type of optical receiver structure, may be referred to as an "integrating" type structure, and is shown diagrammatically in accompanying FIG. 1. This type of structure is commonly employed at relatively low operating frequencies, e.g. up to about 100 Megahertz (MHz).
It comprises an optical receiver 1, e.g. an avalanche or PIN photodiode, associated with a load resistor 2 and an electronic amplifier 3. In order to obtain an acceptable signal to noise ratio, the resistor 2 must have a high resistance, thereby unavoidably creating, in conjunction with the input capacitance to the amplifier 3, a cutoff pole of a lowpass filter. It is therefore conventional to compensate this pole by means of an equalizer stage 4, for moving said pole above the frequency band to be transmitted. The stage 4 is conventionally constituted by a common-emitter connected transistor.
Such a circuit is not suitable for very wide band optical receivers, since it would require too high a number of equalizer stages in cascade in order to be able to operate at frequencies at or above 1 GHz. Furthermore, a circuit including a plurality of equalizer stages in cascade is very difficult to adjust, since each time an equalizer is adjusted, the immediately preceding equalizer must also be adjusted, thereby making such a circuit unsuitable for mass production. Finally, if such a circuit is to operate at high frequencies, it is incapable of transmitting low frequency service channels since it saturates at low frequencies. A last point is that such a circuit is very temperature sensitive, since it does not include any regulator means responsive to temperature.
A second conventional type of optical receiver structure may be referred to as a "transimpedance" structure, and an example is shown diagrammatically in accompanying FIG. 2. One example of a receiver of this type is the so-called "PINFET RECEIVER MODULE" sold by the American LASERTRON corporation. As shown, it comprises a photodiode 5, e.g. a PIN diode, an amplifier 6 which may comprise a field effect transistor (FET) followed by a transistor constituting a cascode type first stage, and an impedance-matching output stage comprising a common-collector connected transistor 7 using an output resistor 8. Finally, and above all, the circuit includes a negative feedback resistor 9 connected between the output from the transistor 7 and the input to the amplifier 6.
Such a transimpedance circuit operates satisfactorily up to frequencies which may be as high as several hundred MHz. Up to such frequencies it operates without any need to add an equalizer stage, and does not suffer from saturation problems at low frequencies or from bad temperature behavior,
However, it is not presently known how to make such a transimpedance structure operate at very high frequencies since, with this known circuit, it is not possible to provide a negative feedback resistance 8 which is high enough, at such frequencies. to obtain an acceptable signal to noise ratio. given that the limiting feature is inadequate gain in the amplifier 6, and that it is not known how to increase this gain without increasing the number of amplifier stages to a quantity which is prohibited at such high frequencies.
It is recalled that the maximum negative feedback resistance Rmax of a transimpedance circuit is related to the gain A of its amplifier, to its stray capacitance Cp, and to the maximum frequency f to be transmitted, by an equation of the form: ##EQU1##
For low gain A, the value of the resistance Fmax is also low, and this means that it is not high enough to obtain a sufficiently high signal to noise ratio. By way of numerical example, for a passband of 1 GHz or or more between 3 dB points, a feedback resistance of at least one kilohm must be used, and this means that the gain A must be at least 30. Unfortunately, it is not known at present how to make an amplifier having a gain of 30 at such frequencies.
Experimental circuits currently exist which, by using very carefully selected components, manage to make do with a feedback resistance of about 600 ohms; but in order to make such a receiver operate with a passband of 1 GHz it is necessary to add common-emitter equalizer stages thereto, thereby returning to the problem of very long and difficult adjustment of said stages, and thus making this type of circuit unsuitable for large scale industrial manufacture.
Preferred embodiments of optical signal receivers in accordance with the present invention avoid the drawbacks of previously known circuits, and are capable of conveying a very wide passband, for example a passband between 3 dB points of more than 1 GHz while still being suitable for large scale industrial manufacture by virtue of the fact that no particularly long or difficult adjustment is required.